Caffeine is considered the most widely used pharmacologic substance in the United States, yet little research has been done on its cardiovascular effects at rest and during psychological and exercise stress. The present proposal describes studies designed to examine how caffeine (3.5 mg/kg, equal to 2-3 cup of coffee) affects cardiovascular function in men at low risk of developing hypertension and those showing early-to-mild hypertension (EH, defined as resting blood pressures of 140-160/90-95). Our research has shown that this dose of caffeine elevates blood pressure at rest by increasing the state for constriction of the peripheral blood vessels. We have also shown that during work on a demanding psychomotor task (unsignalled, simple reaction time with monetary bonuses), caffeine synergistically enhances heart rate, cardiac output, myocardial contractility, and indices of cardiac work. In normotensive men at high risk of developing essential hypertension, caffeine pressor effect in additive with the already elevated blood pressure. As a result, during psychological and aerobic exercise stress, this high risk group shows enhanced levels of blood pressure. At some point during aerobic exercise, 55% of the high risk subjects showed pressures in excess of 230/100 mmHg, considered to be abnormally high during exercise. The proposed studies will extend our work from men at risk to those showing true EH. Study I concerns caffeine's effects on responses to repeated psychomotor challenges separated by brief rests insufficiently long to allow recovery of baseline to occur. Study II tests caffeine's effects on continuous psychomotor and mental challenge with no rests. Study III examines caffeine's potential for producing excessive (greater than or equal to 230/100 mmHg blood pressure responses during graded aerobic bicycle exercise up through maximum exercise. Our use of impedance and nuclear imaging techniques for determining stroke volume and cardiac output permits an extensive characterization of the hemodynamic responses occurring during states of stress under caffeine. This research is of practical significance in providing useful information on caffeine's risk potential in hypertensive men.